1. Technical Field
The present invention relates to a varistor composition. More particularly, the present invention relates to a ZnO-based varistor composition including zinc oxide (ZnO) as a main component.
2. Description of the Related Art
A ZnO-based varistor, which includes a ZnO—Bi-based varistor and a ZnO—Pr-based varistor, is excellent at protecting electronic appliances from surge current because it has excellent voltage nonlinearity and a good ability to withstand surge current compared to a SiC-based varistor or a SrTiO3-based varistor. Therefore, a ZnO-based varistor is generally used as a surge protective device.
The ZnO—Bi-based varistor includes Bi2O3, Sb2O3, Mn, Co, Ni, Cr, glass frit, Al, K and the like. However, since Bi2O3 has low resistance to ESD (electro-static discharge), the ZnO—Bi-based varistor including the same also has low resistance to ESD (electro-static discharge). Further, since the concentration of Sb2O3 is regulated because Sb2O3 is a carcinogen, the production of the ZnO—Bi-based varistor including the same is also regulated. Meanwhile, the ZnO—Pr-based varistor has good voltage nonlinearity, but has large leak current compared to the ZnO—Bi-based varistor. Further, since the ZnO—Pr-based varistor contains Pr which is a precious metal, it must be sintered at high temperature (1200° C. or more). Further, since the ZnO—Pr-based varistor includes a large amount of high-priced components (Pd and the like), its production cost is high.
Meanwhile, recently, as electronic devices became light, thin, small and highly-functional, electronic components have been densely surface-mounted on an electronic device. In the electronic device surface-mounted with electronic components, the signal speed of a circuit is several MHz to several GHz, which is a rapid signal speed. In this rapid signal speed, in order to prevent signal delay, the capacitance of a varistor must be lowered, and, if necessary, may be lowered to 1 pF or less.
Since the ZnO-based varistor has a high specific dielectric constant of several hundreds, the area of an electrode must be reduced in order for the ZnO-based varistor to have low capacitance. However, when the area of an electrode is reduced to impart the ZnO-based varistor with low capacitance, surge resistance is lowered, and the manufacturing process thereof becomes complicated. Therefore, it is required to develop a ZnO-based varistor having a low dielectric constant.